Degradation of the adhesion between rubber and steel or wire reinforcement material is a decades old problem that has been magnified by the growth in steel reinforced pneumatic automobile and truck tires. Tires which are run in an overloaded condition, large size tires, tires run at high speeds, and tires subject to cutting in the tread and or sidewall are particularly susceptible to failure between the rubber compound used to coat the steel and the steel itself. Steel, or steelcord or wire, as used herein refer to the fine strands of brass plated extruded steel twisted together in various configurations and used under the tread area, in the beads, and or in the main body of the tire itself to give added strength to the tire. Skim compounds as used herein refer to the compounds that are utilized to coat the steel, steelcord, wire or fabric reinforcing material in tires. Skim compounds as used herein also refers to the compounds that are utilized to coat the steel, steelcord, wire or fabric reinforcing material utilized in other applications, such as industrial rubber applications which include v-belts, conveyer belts, hoses and the like.
The following terms, known to those of ordinary skill in the tire art, are also used herein. Tread ply refers to a steel or fabric reinforcing member found just under the tread and extending only under the tread area of the tire. Carcass plies are parallel longitudinal reinforcing members which are wrapped around the beads and which may also be steel or fabric. Carcass refers to all tire components excluding the tread, tread ply, undertread, and sidewall, but including the beads, carcass ply (plies), and various ancillary rubber components.
The adhesion problem between the steel, steel cord or wire, and the skim compound, in heretofore known technologies, is also aggravated by constructions which result in cut ends of the steel material coming into direct contact with the rubber compounds as those ends are devoid of the brass plating normally used to enhance the adhesion process. In tires, loss of adhesion between the compound and the steel can result in premature tire failure at the worst, reduced retreadability, uneven treadwear, and a non uniform ride condition (thump and balance).
Adhesion between rubber and steel or wire reinforcements is particularly important in the area of the retreadability of steel truck tires, which are commonly made with steel reinforced treads and carcasses. The economic life of such fires depends on their ability to be retreaded multiple times. One of the major causes of non-retreadability and of failure of the retread on the road is adhesion caused by corrosion of the reinforcing steelcord. Tire manufacturer literature even specifies that the air used to inflate the tire be run through desiccants to insure that it has a very low moisture level.
The presence of moisture in the skim compound itself has been shown to be particularly detrimental to wire adhesion, resulting in rust formation (oxidation of the steelcord) and rapid loss of adhesion. Tires run in snowy areas have been shown to be particularly prone to early rusting and failure due to the presence of salt on the roads. The highly corrosive water and salt solution present on the roads can enter the carcass of the tire through small cuts or cracks in the tread and sidewall area.
Prior art has also shown that moisture in rubber skim compounds leads to degradation of the cure system and generation of amines. This results in non uniform cure rates which give non uniform tensile, modulus, elongation, tear strength and flex resistance. The corrosive nature of amines is particularly troublesome when they are generated in a compound which coats polyester fabric or steel reinforcing material of a tire, either in the tread ply, carcass ply, or bead area, or any other application incorporating the skim compound.
The rubber industry has developed several tests to measure the moisture absorbing tendencies of the skim rubber compounds. These tests subject the cured rubber/steel or rubber/fabric test piece to accelerated moisture exposure, including high humidity, immersion in water, exposure to steam, and immersion in a brine solution. These tests attempt to duplicate the most troublesome service conditions and ensure that the skim compounds and steelcord and fabric constructions chosen maximize the resistance to the detrimental effects of moisture.
Because of industry wide problems in this area, including one of the largest passenger tire recalls in industry history, much work has been done on the adhesion mechanism. The content and thickness of the brass plating on the steelcord, the cure system of the rubber compound, and the presence of certain additives such as cobalt have been shown to improve the adhesion level to a point that results in acceptable product performance in certain cases.
The present invention advantageously provides another method for minimizing the loss of adhesion between the skim compound and the steel, steelcord, wire or fabric reinforcing material. The present invention may be utilized alone, or in conjunction with prior methods for minimizing the loss of adhesion.